Creatine Kinaase (CK; E.C. 2.7.3.2) activity was detected in the lysates of human sperm and in the seminal fluid. By electrophoretic and immunoinhibitory studies, the CK activity appears to be related to the occurrence of CK-BB isoenzyme in both sperm and seminal fluid. The effect of CK-BB antibodies on sperm motility and viability suggest that abnormalities in CK activity may be a cause of unexplained infertility.